[ticket/12856] Copy plupload images to prosilver dir

PHPBB3-12856
[ticket/12856] Remove images from assets/plupload/img/

PHPBB3-12856
Merge branch 'develop-ascraeus' into develop

* develop-ascraeus:
  [ticket/12856] Remove images from assets/plupload/img/
  [ticket/12856] Copy plupload images to prosilver dir